1. Field of the Invention
The present invention relates generally to an amplitude modulation (AM) stereo receiver and, specifically, is directed to an AM stereo receiver for receiving and demodulating AM stereo signals broadcast according to various AM stereo broadcast schemes.
2. Description of the Prior Art
There have been several systems proposed for use in transmitting and receiving AM stereo radio broadcasts. All of the various systems were studied by the Federal Communications Commission (FCC) for approval, however, the FCC did not select any one system but approved five different AM stereo systems. The decision as to which system would ultimately prevail has been left to the marketplace. This has resulted in no one system coming to the fore as of yet, since no individual manufacturer has been willing to select one of the various AM stereo systems and make the investment necessary to produce receivers in quantity which could then receive signals broadcast according to only one of these different AM stereo systems. Presently the five most popular AM stereo systems are as follows:
(1) The AM-PM system, in which the carrier is amplitude modulated by a sum signal (L+R) made up of the stereo left and right signals L and R, respectively, and the carrier is then phase modulated by a difference signal (L-R) made up of the difference between the stereo left and right signals, respectively. This AM-PM system is described in U.S. Pat. No. 4,302,626.
(2) The AM-FM system, in which the carrier is amplitude modulated by the sum signal (L+R) and is also frequency modulated by the difference signal (L-R). This AM-FM system is described in U.S. Pat. No. 3,068,475.
(3) The C-QUAM (Compatible Quadrature Modulation) system in which two carriers that are of the same frequency but differ in phase by 90.degree. are balance modulated with the left and right channel signals L and R, respectively, and are then added to each other to provide a phase modulation signal, which is then amplitude modulated by the sum signal (L+R). The C-QUAM system is described in U.S. Pat. No. 4,218,586.
(4) The VCPM (Variable-Angle Multiple Channel Modulation) system, in which orthogonal modulation is employed wherein the phase angle difference is controlled in response to the amplitude of the difference signal (L-R) from the left and right stereo signals. The VCPM system is described in U.S. Pat. No. 4,225,751.
(5) The ISB (Independent Side Band) system in which the sum and difference signals (L+R) and (L-R), respectively, of the stereo left and right channel signals are orthogonally modulated and are then passed through phase shifting circuits for phase shifting plus or minus 45.degree. to form the signal of the ISB system. The ISB system is described in U.S. Pat. Nos. 3,218,393 and 4,018,994.
As seen from the above, these various systems are relatively complex but, more importantly, are all substantially different from each other, accordingly, there has not been available a receiver that can receive the AM stereo signals broadcast according to these different kinds of modulation schemes. That is, every present AM stereo receiver receives only a single kind of AM stereo broadcasts.